Process for biological removal of sulphide

ABSTRACT

A process for the removal of sulphides, including hydrogen sulphide, carbonyl sulphide and carbon disulphide, from a gas stream by scrubbing the gas with an aqueous washing liquid and treating the washing liquid with sulphide-oxidizing bacteria in the presence of an electron acceptor and reusing the treated liquid as a washing liquid, wherein the scrubbing and the bacterial treatment are carried out in the same reactor and nitrate is used as the electron acceptor. The process is especially useful of desulphurising gas at high pressure, such as natural gas.

CROSS REFERENCE TO RELATED APPLICATION

This is the 35 USC §371 national phase of international applicationPCT/NL97/00647 filed on Nov. 26, 1997, which designated the UnitedStates of America.

FIELD OF THE INVENTION

The present invention relates to a process for the removal of reducedsulphur compounds from a gas stream by scrubbing the gas with an aqueouswashing liquid and treating the spent washing liquid withsulphide-oxidising bacteria in the presence of an electron acceptor in areactor and recycling the treated liquid as a washing liquid.

BACKGROUND OF THE INVENTION

Such a process in known e.g. from WO 92/10270, WO 94/29227 and WO96/30110. In these prior art processes oxygen is used as an electronacceptor. The oxygen is fed at a limited rate so as to direct theoxidation of sulphide to elemental sulphur rather than to sulphate. Theuse of oxygen as an electron acceptor, however, requires the presence ofan aeration system including a compressor, pipings, spargers, and inmost cases a separate reactor. The investment costs for such a systemare relatively high, especially when small volumes of water or gas areto be treated or when e.g. high-pressure natural gas is to bedesulphurised.

SUMMARY OF THE INVENTION

According to the invention, the spent washing liquid containing thereduced sulphur compounds is treated with sulphide-oxidising bacteria inan integrated scrubber/bioreactor, using nitrate as an electronacceptor.

In the present context, the term “reduced sulphur compound” isunderstood to comprise any gaseous or volatile sulphur compound whereinsulphur has the oxidation state-2. Such compounds include hydrogensulphide, lower alkyl mercaptans such as methane-thiol, lower alkylsulphides and disulphides such as dimethyl sulphide, carbonyl sulphide(COS) and carbon disulphide (CS₂). Especially relevant are H₂S, COS andCS₂.

The biological oxidation reactions of hydrogen sulphide by colourlesssulphur bacteria, such as the genus Thiobacillus, especially the speciesT. denitrificans, using nitrate as an electron acceptor are thefollowing:

H₂S+OH⁻→HS⁻+H₂O  (1)

H₂S+HCO₃ ⁻→HS⁻+H₂O+CO₂  (1a)

5HS⁻+2NO₃ ⁻+H₂O→5S°+N₂+7OH⁻  (2)

5HS⁻+8NO₃ ⁻+H₂O→5SO₄ ² ⁻+4N₂+4N₂+3OH⁻+H₂O  (3)

5H₂S+2NO₃ ⁻→5S°+N₂+2OH⁻4H₂O  (1+2)

5H₂S+8NO₃ ⁻+2OH⁻→5SO₄ ² ⁻+4N₂+6H₂O  (1+3)

Reaction (1) denotes a preliminary reaction, e.g. occurring in a gasscrubber, wherein gaseous hydrogen sulphide is dissolved ashydrosulphide anions. Reaction (2) describes the anoxic oxidation ofsulphide to elemental sulphur, whereas reaction (3) represents thecomplete oxidation of sulphide to sulphuric acid. Reaction (1+2) is thetotal net reaction of hydrogen sulphide to elemental sulphur.

Carbonyl sulphide and carbon disulphide may hydrolyse to hydrogensulphide according to the following reactions or their anionicequivalents:

COS+H₂O→H₂S+CO₂  (4)

CS₂+2H₂O→2H₂S+CO₂  (5)

Alternatively or additionally, COS and CS₂ may be oxidised directly,according to the following reactions;

5COS+2NO₃ ⁻+H₂O→5CO₂+5S⁰+N₂+2OH⁻  (2′)

5COS+8NO₃ ⁻+H₂O→5CO₂+5SO₄ ²⁻+4N₂+2OH⁻  (3′)

5CS₂+4NO₃ ⁻+2H₂O→5CO₂+10S⁰+2N₂+4OH⁻  (2″)

5CS₂+16NO₃ ⁻+2H₂O→5CO₂+10SO₄ ²⁻+8N₂+4H⁺  (3″)

The reactions involving oxidation of thiosulphate with nitrate aselectron acceptor are the following:

5S₂O₃ ²⁻+8NO₃ ⁻+2OH⁻→10SO₄ ²⁻+4N₂+H₂O  (6)

5S₂O₃ ²⁻+2NO₃ ⁻+H₂O→5S°+5SO₄ ²⁻+N₂+2OH⁻  (7)

Nitrate can be added as a solid salt, but preferably it is added as aconcentrated solution of e.g. potassium nitrate, or a mixture of anitrate salt and nitric acid. As the conversion of H₂S and other reducedsulphur compounds to elemental sulphur produces alkali (equation1+2/2/′/2″), and the conversion of H₂S and other reduced sulphurcompounds to sulphate consumes the same amount of alkali (equation1+3/3′/3″), acid (preferably nitric acid replacing part of the nitrate)should be added in the preferred case where sulphide is predominantlyconverted to sulphur. Preferably, nitrate (and nitric acid) is added ina substantially stoichiometric amount for oxidation of reduced sulphurcompound predominantly to sulphur, i.e. about 0.4 mole of nitrate permole of H₂S or COS, optionally allowing for minor oxidation to sulphate,i.e. 0.4-0.9, especially 0.4-0.6 mole of nitrate per mole of H₂S or COS,and the double amount for CS₂. An overdosis of nitrate should beavoided, because it destabilises the process due to an accumulation ofnitrite (NO₂ ⁻). The nitrite concentration should remain below 1 mM,preferably below 0.5 mM.

The nitrate addition can be controlled using the redox potential of theaqueous solution, as described in WO 98/04503. Thus the redox potentialof the medium of the oxidation is adjusted at a value below −150 mV(against an Ag/AgCl reference electrode), especially below −250 mV. Thepreferred redox potential range is form −300 to −390 mV, more preferablyfrom −320 to −360 mV (against an Ag/AgCl reference electrode). The rangeof −300/−390 mV against Ag/AgCl corresponds to a range of −97/−187 mVagainst a H₂ reference electrode at 30° C. The redox setpoint valuesapply for a temperature of 30°C. and a pH of 8.

The temperature of the biological oxidation is between 10 and 85° C.,the optimum being between 20 and 50° C., especially at about 30° C. Theoptimum pH is in the range of 7-9. If the solution does not containnutrients, as is the case with gas scrubbing, these have to be suppliedas well, This can be done at the same time as the nitrate supply. Theelectric conductivity of the washing liquid is preferably kept between30 and 100 mS/cm.

The bacteria to be used in the present process can be taken fromconventional sulphide-oxidising cultures. The bacteria are preferablyneutrophilic bacteria and will typically include Thiobacillus species,especially T. denitrificans.

The process of the invention can be used for treating gases alsocontaining carbon dioxide. The carbon dioxide contributes to the H₂Sloading capacity of the scrubbing liquid, especially at high pressures.As an example, the H₂S loading capacity of a scrubbing liquid forscrubbing a pressurised gas (95 bar) having a CO₂ content of 1.1 vol. %is 200 to 300 g/m³. Also the carbon dioxide can be used as a carbonsource for the sulphide-oxidising bacteria.

In the process of the invention for the removal of hydrogen sulphide andother reduced sulphur compounds from gas streams, the solution isrecycled after oxidation of the reduced sulphur compounds to elementalsulphur, using the same reactor for scrubbing and for anoxic biologicaltreatment. No liquid recirculation between different pressures isnecessary. Further advantages are that the equipment can be relativelysimple and inexpensive, and that the recycle ratio and thus the liquidresidence time can be high so that any loss of biomass is compensated bybacterial growth. If required, the bacteria can be immobilised on acarrier, which carrier can be combined with a packing material in thescrubber. For a simple operation such immobilisation can be omitted.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE depicts an installation suitable for carrying out theprocess according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The process can be performed in an installation as depicted in theFIGURE. According to this FIGURE, a gas scrubber/reactor 1 is equippedwith a sulphide-containing gas supply 2 and a purified gas exit 3.Washing liquid is recycled through 4 without pressure difference andsurplus liquid is drawn off at 5. The scubber contains a packingmaterial 6 for improving contact between gas and washing liquid. Make-upwater, containing nutrients and trace elements, is introduced throughline 7. Nitrate (electron acceptor) is supplied through 8, but may alsobe added to the make-up water in 7. Nitric acid can be used for pHcontrol and be added through 7 or 8 or separately. A separator 9 isdisposed in bleed stream 5, where a sulphur slurry is separated off anddischarged through 10. The clarified liquid 11 can be reused as make-upwater at 7. As an alternative, a sulphur separator may be placed inrecycle loop 4. Preferably the elemental sulphur is not completelyseparated: a residual level of 0.1-5 wt %, especially 0.3-3 wt. % isadvantageous for enhance sulphide absorption by the washing liquid andimproved buffering. The sulphur level in the scrubber/reactor can beadjusted by adjusting the recycle ration (flow 4 vs. flow 5) and/or byadjusting the separation efficacy of the sulphur separator.

The process of the invention can be advantageously applied of thetreatment of small biogas streams, such as those producing less than 100kg of sulphur per day. For such streams, the cost for the addition ofnitrate is more than compensated by the saving on investment as a resultof the omission of expensive compressors, spargers and other devicesnecessary for oxygen supply.

The process can also be used for treating ventilation air containinghydrogen sulphide, such as generated in anacrobic treatment reactors,pumping house and decanters. Where such venting air containsinsufficient oxygen, or where the transfer of oxygen to the aqueousphase is insufficient, the nitrate suitable acts as an (additional)electron acceptor.

The process of the invention is also suitable for the treatment ofnatural gas and other gas streams of high pressure, e.g. 50 bar orhigher. These high pressures normally make biological treatment ofnatural gas unattractive, because of the high energy consumptionresulting form pressurising the washing liquid for entering the gasscrubber and depressurising the liquid for entering the aerated aerobicreactor. Prior art processes for removal of hydrogen sulphide and otherreduced sulphur compounds from natural gas include (1) reversiblechemical or physical absorption based on alkanolamine or carbonatesolvent; these processed required high temperatures and pressuredifferences; also part of the expensive amine solution is lost with thebleed stream; (2) direct conversion through absorption and oxidatione.g. based on a redox reaction between H₂S and Fe³⁺, wherein the reducedmetal is reoxidised with oxygen; drawbacks of such processes are thetendency of plugging with sulphur solids and the use of expensive,corrosive metal chelates; (3) non-regenerative absorption, whereinhydrogen sulphide is absorbed onto, e.g., active carbon or iron sponges,which are regularly replaced and disposed (“throw away”).

In the present process the pressurising/depressurising steps can beeliminated, since the biological treatment does not require oxygen gasand the gas scrubbing can thus be integrated with the bioreactor.

Other gases that can be treated with the process of the inventioninclude gases associated with oil and natural gas, stabilisation gas,gas from high vacuum units, fuel gas, gas from HDS units, gas frommolecular sieves, syngas, hydrogen sulphide-containing gas from CO₂ gasstreams.

EXAMPLE 1

In an installation as depicted in FIG. 1 having a scrubber volume of 10m³, natural gas at 95 bar containing 100 ppmv of H₂S and 1.1 vol. % ofCO₂ is treated at 25,000 Nm³/day, using a scrubbing liquid having a pHof about 8.5 and using a biomass containing Thiobacillus denitrificans.Potassium nitrate and nitric acid are fed at 50-200 g/hr and 50-200g/hr, respectively. A liquid recycle rate of 0.5-5 m³/hr is used. TheH₂S content of the purified gas is 2 ppmv (desulphrisation efficacy:98%).

EXAMPLE 2

An installation as depicted in FIG. 1, consisting of an integratedscrubber and bioreactor, was used for the removal of H₂S fromventilation air, which contains both H₂S and oxygen. The treated airflowamounts to 900 m³/h with a H₂s concentration between 500 and 800 ppm.The scrubbing liquid was held at a pH between 8.5 and 9 with a specificconductivity of 40 mS/cm. Nitrate was added at 0.3 kg/h from acombination of nitric acid and potassium nitrate such that the pH wasmaintained in the indicated range. The nitrate to sulphide ratio was0.25 mol/mol, which is well below the theoretically expected ratio of0.4-1.6 mol/mol. The reason for this is that the ventilation aircontains oxygen which also acts as an electron acceptor. During thetesting period, no H₂S could be detected in the off-gas, while thenitrate concentration was below 10 ppm. Residual elemental sulphur inthe bleed stream was decanted and recovered.

What is claimed is:
 1. A process for the removal of reduced sulfurcompound comprising at least one component selected from the groupconsisting of hydrogen sulfide, carbonyl sulfide and carbon disulfidefrom a gas stream, the process comprising: scrubbing the gas stream withan aqueous washing liquid to obtain a spent washing liquid; biologicallytreating the spent washing liquid with sulfide-oxidizing bacteria in thepresence of an electron acceptor to oxidize reduced sulfur compounds toelemental sulfur, and obtain a treated liquid; reusing the treatedliquid as a washing liquid; the scrubbing and biological treatment beingcarried out in the same reactor by continuously adding as the electronacceptor a mixture of nitrate salt and nitrate acid at a rate of 04.-0.9mole per mole of hydrogen sulfide or carbonyl sulfide and 0.8-1.8 moleper mole of carbon disulfide.
 2. The process according to claim 1,wherein the elemental sulfur is partly removed form the treated liquidsuch that a residual level of sulfur ranging form 0.1-5wt. % remains. 3.The process according to claim 1, wherein the washing liquid has aspecific conductivity ranging between 30 and 100 mS/cm.
 4. The processaccording to claim 1, wherein neutriphilic sulfide-oxidizing bacteriaare used.
 5. The process according to claim 1, wherein the washingliquid has a pH ranging between 7 and
 9. 6. The process according toclaim 1, wherein the gas stream also contains carbon dioxide.
 7. Theprocess according to claim 1, wherein the gas stream is a gas having apressure of at least 50 bar.
 8. The process according to claim 1,wherein the gas stream is a ventilation gas.
 9. The process according toclaim 1, wherein the addition of nitrate is controlled by maintainingthe redox potential of the aqueous washing liquid at between −300 and−390 mV, against an Ag/AgCl reference electrode.
 10. The processaccording to claim 1, wherein 0.4-0.6 mole of nitrate/nitric acid permole of hydrogen sulfide or carbonyl sulfide and 0.8-1.2 mole per moleof carbon disulfide is added as the electron acceptor.